o i b 48, Saint-Petersburg 191 186, Russia 1
. IntroductionThe most important of the chemical compounds of lead are the lead oxides. Lead oxides are technologically important because they exhibit obvious photoactivity over a wide range of wavelengths; it is for tlus reason that they are used in imagng devices, particularly, in production of television pickup tubes of "Plumbicon" type, electrophotographc plates, and ferromagnetic and ferroelectric materials. Complex crystal structures, unusual polymorphc transformations, as well as nonstoichiometric forms of lead monoxide have attracted the efforts of many researchers for almost half of a century [l-51. The stoichometry of lead oxides was a subject of early investigations and a large number of research papers published by tlus time, where matters were considered related to methods employed when preparing and studyng any possible oxides and homogeneous phases that are intermediate in composition interval from red lead Pb304 up to lead dioxide Pb02. Subject of tlus study are the polarization properties in the tetragonal lead monoxide. The crystallografic features of structure also are considered. Sipficant influence of the PbZ+ electron lone pairs in the area of low temperature phase transitions is supposed to take place.
Peculiarities of PbO crystal structure.Most of the lead oxides are characterized by having a defect structure in a rather wide range of compositions variability. Besides, a suggestion has' been made that it is the method of preparing samples that effects a defect Structure. And this is almost equivalent to sayng that the structure of the lead oxide crystal is a "memory system" that remembers the main aspects of the processing that produced it. At the same time, according to the experimental results [5], lead oxides avidly combine with carbon &oxide and water to g v e a vast assortment of carbonates, basic carbonates, and hydrated products. Therefore, analytical data seelung the nature of a lead oxide product been made must be corrected for these compounds whose formation is difficult to avoid. According to X-ray and neutron Qffraction analysis [l], photoconductive lead monoxide occurs in two polymorphc forms: a tetragonal a-modification stable up to 762.5 K and a yellow orthorhombic p P b 0 form that is stable at elevated temperatures T >'760 K. Both phases have a layered structure (orthogonal to the c axis) and occur naturally as the minerals of litharge and massicot respectively. The tetragonal aphase is stable at pressures below 0.7 GPa, whereas the orthorhombic Pphase is stable for pressures above 2.5 GPa.The a and p structures are characterized by similar paclung of lead atoms (distorted face-centered lattice); they differ mainly by the positions of the oxygen atoms. Another important feature is the following: the interlayer spaces are occupied by the electronic lone pairs of the Pb2+ ions. The crystal structure of tetragonal a-PbO consists of layers of alternately regular square pyramids Pb04 in a tetragonal lattice (P4/nmm; at=bt=3.9704 A; c...
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